Effect of desilication on the textural properties, acidity and catalytic activity of zeolite ZSM-23 synthesized with different structure-directing agents

The synthesis of ZSM-23 zeolite was studied using isopropylamine, ethylene glycol, pyrrolidine and N,N-dimethylformamide as structure-directing agents, followed by post-synthesis treatment. The samples were characterized by X-ray diffraction (XRD), dispersive energy X-ray spectroscopy (EDX), thermal analysis, adsorption-desorption of nitrogen, temperature programmed desorption of ammonia (TPD-NH3) and scanning electron microscopy (SEM). ZSM-23 was obtained with all organic molecules tested, and the sample synthesized with isopropylamine presented a higher crystallinity. EDX measurements indicated parent zeolites with SiO2/Al2O3 ratio ≥66. Thermal analysis curves showed that the parent sample prepared with isopropylamine presented lower mass loss, corroborating the crystallinity results and indicating a greater efficiency of this compound in the ZSM-23 synthesis. Micrographs of the parent samples synthesized with isopropylamine, ethylene glycol and N,N-dimethylformamide illustrated the characteristic morphology of ZSM-23 as “needles”, whereas the sample obtained with pyrrolidine showed the coexistence of ZSM-23 and ZSM-5 phases. TPD-NH3 results demonstrated that the post-synthesis treatment promoted, in all cases, a decrease in the weak acid sites and an increase in the strong and total acid sites densities for the samples from the isopropylamine and N,N-dimethylformamide series. Adsorption-desorption of nitrogen showed an increase in pore volume and specific surface area values for the modified samples compared to the parent materials. Modified zeolites presented higher catalytic activity and better deactivation resistance in n-heptane catalytic cracking in comparison to the precursor samples, with all catalysts showing excellent selectivity for propene and ethene.